Recently, the demand for liquid crystal panels has increased not only for direct-view liquid crystal panels but also for projection-type display devices such as a projection T.V. A liquid crystal panel is required to have sufficient brightness when it is used as a projection-type display device. In order to improve the brightness of a liquid crystal panel, the numerical aperture of the pixels should be improved. As a conventional method to improve the numerical aperture, providing microlenses to one of the substrates of a liquid crystal panel is well known, as disclosed in Publication of Japanese Patent Application (Tokkai Hei) No. 3-248125.
A conventional liquid crystal panel having microlenses is manufactured in the following steps.
First, on a first transparent substrate provided with fine microlenses, a second transparent substrate is adhered with an adhesive in order to form a lens substrate (hereinafter, a microlens array substrate). The second transparent substrate is a plate of glass, plastic and so on, and black matrices are provided as a shielding layer on this second transparent substrate. The ordinary thickness of the microlens array substrate is 1100 .mu.m, or the thinnest one is 600 .mu.m.
Next, a thin film transistor substrate provided with plural pixels and the microlens array substrate are adhered to each other at the peripheral regions with a seal adhesive while keeping a predetermined distance between these substrates, and liquid crystal is filled between them so that a liquid crystal panel is completed. Each microlens corresponds with each pixel provided for the thin film transistor substrate.
If a liquid crystal panel having no microlens is irradiated, the light is partially shielded by the black matrices. On the other hand, if a liquid crystal panel having microlenses to condense light is irradiated with light, the light that otherwise would be shielded by the black matrices is condensed at the apertures having no black matrices formed thereon. Therefore, a liquid crystal panel having microlenses can increase substantially the light condensing amount compared to the panel having no microlenses.
Actually, the light condensed by the microlenses is not condensed at one point on the pixel due to the angular dispersion of the light entering the liquid crystal panel, but it forms a light condensing spot having a certain range. So the condensing spot should be smaller than the pixel in order to condense light effectively by using the microlenses. A small condensing spot can be obtained by shortening the focal distance of the microlens. The condition to obtain the condensing effect is indicated by the following formula: EQU d=2f tan .theta.,d&lt;1
As shown in FIG. 3, d is the diameter of a condensing spot, f is the focal distance of a microlens, .theta. is a dispersion angle of the light entering the liquid crystal panel, l.sub.1 is the aperture size of a pixel l, and l.sub.2 is the size of the shielding part of the pixel l.
As a recent projection-type display device is desired to have a high-definition liquid crystal panel, pixels as small as 15 .mu.m are formed. The dispersion angle .theta. of the light entering the liquid crystal panel ranges from 5 to 10 degrees in general. Therefore, the focal distance f of the microlens should not exceed 43 .mu.m, considering the above formula. Black matrices are provided at the focal points of the microlenses. Therefore, the total length consisting of the thickness of the second transparent substrate and the gap between the first and second transparent substrates (that is, the thickness of the adhesive layer) should not exceed 43 .mu.m, namely, the focal distance f of the microlens.
In a general method to adhere microlenses on a transparent substrate, the microlenses are inserted between the parallel transparent substrates and adhered while being pressed. In this method, however, it is difficult to provide a thin adhesive layer and to make the gap between the two transparent substrates uniform as a whole. When the adhesive layer is thick, the gap between the transparent substrates tends to be further nonuniform. In a conventional method, the parallel precision between the transparent substrates is generally 3-4 .mu.m. Moreover, even one foreign matter at least several .mu.m thick between the transparent substrates will prevent the adhesive layer from having a uniform thickness. Therefore, extremely precise and clean equipment for adhesion is required.
In manufacturing a liquid crystal panel, when the thickness of an adhesive layer that adheres a first transparent substrate having microlenses and a second transparent substrate is nonuniform, the distance from the microlenses to the black matrices becomes nonuniform. And the light condensing amount of the microlenses will be nonuniform at the respective pixels, resulting in display nonuniformity. In addition, the surfaces of the transparent substrates will have waves because of the thickness nonuniformity of the adhesive layer. Therefore, the liquid crystal thickness will be nonuniform, and further display nonuniformity will occur. In order to prevent such display nonuniformity, nonuniformity in the thickness of the adhesive layer should be controlled to be about 1 .mu.m.